Mineral wool vs Rockwool vs PIR: which should you pick?

Three names come up again and again when you're choosing insulation: mineral wool, Rockwool and PIR boards. They do the same basic job — slowing heat loss — but in quite different ways, at different prices and with different strengths. Here's how to tell them apart and pick the right one for your home.

Mineral wool: the cost-effective all-rounder

Mineral wool (glass wool) is the soft, fluffy roll most people picture when they think of loft insulation. It's inexpensive, easy to fit between and over joists, and widely available, which makes it the default choice for an open loft floor. The trade-off is bulk: it needs a fair thickness to hit the recommended performance, so it's less suited to spaces where headroom is tight.

• Best for: standard loft floors and budget-conscious upgrades.
• Strengths: low cost, easy to install, good all-round performance.
• Watch out for: it's bulky and can irritate skin during fitting.

Rockwool: stone wool with fire and sound benefits

Rockwool is a type of mineral wool made from stone (often called stone wool or slab). It's denser and more rigid than soft glass-wool rolls, and it's particularly valued for its excellent fire resistance and sound-deadening. That makes it a strong choice where acoustic comfort or fire performance matters — for example between floors, in stud walls, or around a home, as well as in the loft.

• Best for: fire performance, soundproofing and more robust applications.
• Strengths: excellent fire resistance, good acoustic properties, dense and durable.
• Watch out for: typically costs more than standard glass-wool rolls.

PIR boards: high performance in a slim profile

PIR (polyisocyanurate) boards are rigid foam panels — the familiar foil-faced boards from brands like Celotex and Kingspan. Their big advantage is performance-per-millimetre: they achieve the same thermal result as mineral wool in a much thinner layer. That makes them ideal for loft conversions, sloped ceilings, boarded loft floors and anywhere space is at a premium.

• Best for: loft conversions, warm roofs, boarded floors and tight spaces.
• Strengths: highest performance for the least thickness, rigid and boardable.
• Watch out for: it's the priciest of the three and needs careful cutting and sealing.

What about multifoil?

You'll also come across slimline multifoil and SuperFOIL insulation, which reflects radiant heat and is often combined with the materials above where headroom is limited. It's worth considering alongside, rather than instead of, these three.

Quick comparison

• Cheapest: mineral wool.
• Best fire & sound: Rockwool.
• Best performance in least space: PIR boards.
• Best for tight spaces / rafters: multifoil, often in combination.

The bottom line

There's no single winner — the right material depends on your loft, your budget and whether the space is used for storage, a conversion or living. A good installer will often combine materials to get the best result. Tell us about your home and we'll advise on the right mix, then match you with vetted insulation installers so you can compare quotes.